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I've had a fair bit of success with mega banette, but its one of the most serious cases of 4 moveslot syndrome I've ever encountered.
To get the best out of mega banette, you need to decide what you want it to do. It can stall break with priority taunt and disable, which is the thing most people expect from it, and it can hit pretty hard with EVs in attack and STAB moves like shadow claw, but the thing I've found it really amazing for is abusing the existence of priority destiny bond.
Anyone who knows what mega banette has access to will know that there simply isn't a safe way to attack it unless you're willing to lose the attacker. Because of this, I've found it to be amazing at blocking sweeps, by simple virtue of it having access to destiny bond. someone can invest a lot of effort in getting their sweeper to +4 but it doesn't mean shit if there's a mega banette in front of it.
The other, less often trumpeted option is its ability to fuck up baton pass teams if things are getting out of hand. Curse is not blocked by a substitute, IS passed on by baton pass, and thanks to prankster, is guaranteed to happen if you carry protect. Giving up half your hp might sound like a bad deal, but if you end up staring down a dedicated pass team and they get several buffs up, its definitely worth it, especially if you have something that can pass it a wish.
It can't do everything at once, so breed a really good one with all the egg moves you might ever need and play around with movesets. The biggest gift to banette is the fact that these days you can use a heart scale to remember egg moves you bred it to have, so you can breed it to have destiny bond/disable and ditch them when they aren't what you want at the time.
Banette's not a sweeper (unless you give it max attack, shadow sneak, and come up against a team of ghosts), but its one of the best "fuck you" buttons in the game.